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<title>17.4.2 Flow Control Issues </title>
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<h2><a name="SECTION0019420000000000000000"></a><a name="popen2-flow-control"></a>
<br>
17.4.2 Flow Control Issues 
</h2>

<p>
Any time you are working with any form of inter-process communication,
control flow needs to be carefully thought out.  This remains the case
with the file objects provided by this module (or the <tt class="module"><a href="module-os.html">os</a></tt>
module equivalents).

<p>
When reading output from a child process that writes a lot of data to
standard error while the parent is reading from the child's standard
output, a deadlock can occur.  A similar situation can occur with other
combinations of reads and writes.  The essential factors are that more
than <tt class="constant">_PC_PIPE_BUF</tt> bytes are being written by one process in
a blocking fashion, while the other process is reading from the other
process, also in a blocking fashion.

<p>
There are several ways to deal with this situation.

<p>
The simplest application change, in many cases, will be to follow this
model in the parent process:

<p>
<div class="verbatim"><pre>
import popen2

r, w, e = popen2.popen3('python slave.py')
e.readlines()
r.readlines()
r.close()
e.close()
w.close()
</pre></div>

<p>
with code like this in the child:

<p>
<div class="verbatim"><pre>
import os
import sys

# note that each of these print statements
# writes a single long string

print &gt;&gt;sys.stderr, 400 * 'this is a test\n'
os.close(sys.stderr.fileno())
print &gt;&gt;sys.stdout, 400 * 'this is another test\n'
</pre></div>

<p>
In particular, note that <code>sys.stderr</code> must be closed after
writing all data, or <tt class="method">readlines()</tt> won't return.  Also note
that <tt class="function">os.close()</tt> must be used, as <code>sys.stderr.close()</code>
won't close <code>stderr</code> (otherwise assigning to <code>sys.stderr</code>
will silently close it, so no further errors can be printed).

<p>
Applications which need to support a more general approach should
integrate I/O over pipes with their <tt class="function">select()</tt> loops, or use
separate threads to read each of the individual files provided by
whichever <tt class="function">popen*()</tt> function or <tt class="class">Popen*</tt> class was
used.

<p>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-subprocess.html">subprocess</a></tt>:</b>
    <dd>Module for spawning and managing subprocesses.
  </dl>
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